As a detector for a gas chromatograph, various types of detectors have been practically applied, such as a thermal conductivity detector (TCD), electron capture detector (ECD), flame ionization detector (FID), flame photometric detector (FPD), and flame thermionic detector (FTD). Among these detectors, the FID is most widely used, particularly for the purpose of detecting organic substances. The FID is a device that ionizes sample components in a sample gas by hydrogen flame and detects the resultant ion current. It can attain a wide dynamic range of approximately six orders of magnitude. However, the FID has the following drawbacks: (1) Its ionization efficiency is low, so that its minimum detectable amount is not adequately low; (2) Its ionization efficiencies for alcohols, aromatic substances, and chlorine substances are low; (3) It requires hydrogen, which is a highly hazardous substance; therefore, an explosion-proof apparatus or similar kind of special equipment must be provided, which makes the entire system difficult to operate.
On the other hand, a pulsed discharge detector (PDD) has conventionally been known as a detector capable of high-sensitivity detection of a wide variety of compounds ranging from inorganic substances to low-boiling-point organic compounds (see Patent Document 1 or other documents). In the PDD, the molecules of helium or another substance are excited by a high-voltage pulsed discharge. When those molecules return from their excited state to the ground state, they generate light energy. This light energy is utilized to ionize a molecule to be analyzed, and an ion current produced by the generated ions is detected to obtain a detection signal corresponding to the amount (concentration) of the molecule to be analyzed.
In most cases, the PDD can attain higher ionization efficiencies than the FID. For example, the ionization efficiency of the FID for propane is no higher than 0.0005%, whereas the PDD can achieve a high level of approximately 0.07%. However, the dynamic range of the PDD is not as wide as that of the FID; the fact is that the former is lower than the latter by one or more orders of magnitude. This is one of the reasons why the PDD is not as widely used as the FID.
The most probable constraining factors for the dynamic range of the conventional PDD are the unstableness of the plasma created for the ionization and the periodic fluctuation of the plasma state. To solve this problem, a discharge ionization current detector has been proposed (for example, see Patent Documents 2 and 3), which uses a low-frequency AC-excited dielectric barrier discharge (which is hereinafter referred to as the “low-frequency barrier discharge”) to create a stable and steady state of plasma. The low-frequency barrier discharge is featured by the use of an electrode covered with a dielectric member for generating an electric discharge; such an electrode releases a smaller amount of thermions, secondary electrons and similar particles than metallic electrodes, and therefore, can produce plasma with high stability. The excitation of helium or other elements by a low-frequency high voltage leads to the creation of non-equilibrium plasma at a very low gas temperature (with almost no generation of heat), which suppresses the generation of the gas of impurities due to the heating of the materials in the inner wall of the gas tube, so that the plasma stability is even further improved. The stabilization of the plasma has the effect of stabilizing the ionization efficiency and thereby reduces the noise in the ionization current output. Thus, the ionization current detector using a low-frequency barrier discharge can achieve a high signal-to-noise ratio (S/N).